This reverts commit 69a71a0ec2.
Previously, the only use for spirv-tools was its commands being used
to run tests, but now, spirv-llvm-translator also links against its
libraries. Tests won't be run when cross compiling to a
non-compatible architecture anyway, so to keep spirv-llvm-translator
cross compiling, we should move spirv-tools back to being a build
input so that the libraries can be found. We could try to convince
CMake to use SPIRV-Tools.cmake from the host spirv-tools and
SPIRV-Tools-tools.cmake from the build spirv-tools, but since we never
actually need both when cross compiling it's not worth it.
This fixes the cross build LLVM/SPIRV-LLVM-Translator 16.
a9fd593f39